Visual acuity and the accuracy of the accommodative response.

Previous investigators have observed that some subjects show large amounts of accommodative lag. We hypothesized that less accurate accommodation might be associated with poorer visual acuity and/or smaller pupil sizes. Sixty subjects (30 emmetropes and 30 myopes) aged 20-30 years, participated. All had best-corrected visual acuity of 6/6 or better [mean = -0.10 +/- 0.07 logarithm of the minimum angle of resolution (logMAR)]. Subjects monocularly viewed reduced Bailey-Lovie charts through a +6.50 D Badal lens on a Canon R1 auto-refractor. Visual acuity, accommodative response and pupil diameter were measured for 0, 2 and 4 D accommodative stimuli. For accommodation measurements (N= 10) subjects were instructed to fixate the smallest letters that they could read. The mean accommodative response was +0.22 +/- 0.28, +1.83 +/- 0.23 and +3.71 +/- 0.27 D for the 0, 2 and 4 D stimuli, respectively. The mean visual acuity was -0.06 +/- 0.10, -0.11 +/- 0.07 and -0.11 +/- 0.07 logMAR for the 0, 2 and 4 D stimuli, respectively. Visual acuity for the 0 D stimulus was significantly poorer than for other conditions (p < 0.001) and associated with increased accommodative lead (p < 0.01). There was also an association between visual acuity and accommodative response (or lag) for the 4 D stimulus (p=0.002). The emmetropes showed significantly better visual acuity than the myopes (p= 0.004). No significant difference was observed in the accommodative response between emmetropes and myopes. Pupil diameter was not associated with the accuracy of the accommodative response (p > 0.17). Increased accommodative lead (0 D stimulus) and accommodative lag (4 D stimulus) are associated with decreased visual acuity. Smaller pupil diameters are not associated with increased accommodative lag.